A Unified Fitting of H i and He ii Lyα Transmitted Flux of QSO HE 2347−4342 with ΛCDM Hydrodynamic Simulations

Reimers²

2006

A&A

Aims. We present a reanalysis of the He ii Lyα absorption towards the quasars HS 1700+6416 and HE 2347-4342 using new high S /N, optical observations.Methods. An alternative analysis method is applied, which fits the high quality, optical H i data directly to the He ii spectrum. The results are compared to those inferred from standard line profile analyses. This new method enables us to derive redshift scales characterizing the fluctuations of the column density ratio η. Results. We find η changing smoothly with redshift on typical scales of ∆z ∼ 0.01−0.03 corresponding to 8−24 h −1 Mpc comoving. The real length scales of variations of the column density ratio might be even larger, since part of the fluctuations may be caused by noise in the He ii data and by systematic effects due to the applied method. However, variations on small scales cannot be ruled out completely. For η variations on scales of a few Mpc an amplitude of about ±1.5 dex cannot be excluded. The data shows an apparent correlation between low η regions and the presence of metal line absorbers, which corresponds to the more general correlation of low η and strong H i absorption. Thermal line broadening is suggested as a probable explanation for this apparent correlation, since both fit methods would severely underestimate η for absorbers with N H i > ∼ 10 13 cm −2 if the line width was dominated by thermal broadening. Indeed, lines located close to the cut-off of the b(N) distribution yield lower column density ratios compared to the whole sample, in particular if high density absorbers are considered. We argue that the apparent correlation of η with the strength of the H i absorption is caused by insufficient consideration of thermal broadened lines by the standard analysis. As unbiased value of the column density ratio, we find η ∼ 80 in agreement with previous estimates.

Section: Discussionsupporting

confidence: 55%

“…Telfer et al 2002) and finite lifetimes, local density variations of the IGM (Miralda-Escudé et al 2000) or filtering of radiation by radiative transfer effects (Maselli & Ferrara 2005). Furthermore, it has been shown that also the noise level of the observed spectra contributes to the scatter in η (Fechner et al 2006a;Liu et al 2006). …”

Section: Introductionmentioning

confidence: 99%

Fluctuations of the intergalactic UV background towards two lines of sight

Reimers²

2006

A&A

“…Thermal broadening becomes important in collapsed structures at high column densities. In simulations of the Lyα forest, non-thermal broadening has been found to dominate (Zhang et al 1995(Zhang et al , 1998Hernquist et al 1996;Weinberg et al 1997;Bolton et al 2006;Liu et al 2006). This has been confirmed observationally for the low-column density forest (Z04; Rauch et al 2005).…”

Section: Uncertainties In the Spectral Hardnessmentioning

confidence: 99%

The transverse proximity effect in spectral hardness on the line of sight towards HE 2347–4342

Worseck

Wisotzki

et al. 2007

A&A

We report the discovery of 14 quasars in the vicinity of HE 2347−4342, one of the two quasars whose intergalactic He ii forest has been resolved with FUSE. By analysing the H i and the He ii opacity variations separately, no transverse proximity effect is detected near three foreground quasars of HE 2347−4342: QSO J23503−4328 (z = 2.282, ϑ = 3. 59), QSO J23500−4319 (z = 2.302, ϑ = 8. 77) and QSO J23495−4338 (z = 2.690, ϑ = 16. 28). This is primarily due to line contamination and overdensities probably created by large-scale structure. By comparing the H i absorption and the corresponding He ii absorption, we estimated the fluctuating spectral shape of the extragalactic UV radiation field along this line of sight. We find that the UV spectral shape near HE 2347−4342 and in the projected vicinity of the three foreground quasars is statistically harder than expected from UV background models dominated by quasars. In addition, we find three highly ionised metal line systems near the quasars. However, they do not yield further constraints on the shape of the ionising field. We conclude that the foreground quasars show a transverse proximity effect that is detectable as a local hardening of the UV radiation field, although the evidence is strongest for QSO J23495−4338. Thus, the relative spectral hardness traces the proximity effect also in overdense regions prohibiting the traditional detection in the H i forest. Furthermore, we emphasise that softening of quasar radiation by radiative transfer in the intergalactic medium is important to understand the observed spectral shape variations. From the transverse proximity effect of QSO J23495−4338 we obtain a lower limit on the quasar lifetime of ∼25 Myr.

“…However, this result is based on a limited statistical sample of 8 absorbers. Liu et al (2006) claimed to confirm this finding using hydrodynamic simulations. On the other hand, Fechner & Reimers (2007) showed that statistically, lines with apparent low η values are often dominated by thermal broadening.…”

Section: Send Offprint Requests To: C Fechnermentioning

confidence: 80%

“…This is due to the fact that any line profile model recovers the observed data only imperfectly. Additionally, the high noise level of the FUSE data introduces further uncertainties (see also Liu et al 2006). Since these problems are inevitable, we expect that our results can be interpreted statistically but conclusions for single absorbers should be considered very carefully.…”

Section: Methodsmentioning

confidence: 91%

The HeII Lyman alpha forest and the thermal state of the IGM

Reimers

2006

A&A

Aims. Recent analyses of the intergalactic UV background by means of the He  Lyα forest assume that He  and H  absorption features have the same line widths. We omit this assumption to investigate possible effects of thermal line broadening on the inferred He /H  ratio η and to explore the potential of intergalactic He  observations to constrain the thermal state of the intergalactic medium. Methods. Deriving a simple relation between the column density and the temperature of an absorber based on the temperature-density relation T = T 0 (1 + δ) γ−1 we develop a procedure to fit T 0 , γ, and η simultaneously by modeling the observed spectra with Doppler profiles. In an alternative approach the temperature T of an absorber, the He /H  ratio η, and the redshift scale of η variations are estimated simultaneously by optimizing the Doppler parameters of He .Results. Testing our procedure with artificial data shows that well-constrained results can be obtained only if the signal-to-noise ratio in the He  forest is S /N > ∼ 20. Additionally, ambiguities in the line profile decomposition may result in significant systematic errors. Thus, it is impossible to give an estimate of the temperature-density relation with the He  data available at present (S /N ∼ 5).However, we find that only 45% of the lines in our sample favor turbulent line widths while the remaining lines are probably affected by thermal broadening. Furthermore, the inferred η values are on average about 0.05 dex larger if a thermal component is taken into account, and their distribution is 46% narrower in comparison to a purely turbulent fit. Therefore, variations of η on a 10% level may be related to the presence of thermal line broadening. The apparent correlation between the strength of the H  absorption and the η value, which has been found in former studies assuming turbulent line broadening, essentially disappears if thermal broadening is taken into account. In the redshift range 2.58 < z < 2.74 towards the quasars HE 2347-4342 and HS 1700+6416 we obtain η ≈ 100 and slightly larger. In the same redshift range the far-UV spectrum of HS 1700+6416 is best and we estimate a mean value of log η = 2.11 ± 0.32 taking into account combined thermal and turbulent broadening.